Photoresist based on polycondensates and having an increased resolution for use in 157 nanometer lithography

ABSTRACT

A photoresist includes a polymer which has acid-cleavable groups in its main chain. The polymer can thus be cleaved by acid into short cleavage products which can be removed from the substrate through the use of a developer. The polymer is completely or partially fluorinated, and consequently has an improved transparency to light of short wavelengths.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a photoresist which makes it possible toproduce structures with photolithographic methods having a highresolution.

[0003] Thin layers of photoresists are used for structuringsemiconductor substrates during the production of microchips. Thephotoresists can be selectively changed in their chemical nature by anexposure with a photo mask or by a direct exposure, for example by anexposure with an electron beam. After a development step in which theexposed parts of the photoresist are removed, a structured photoresistis obtained and is used as a mask, for example for etching thesemiconductor substrate. A number of processes have already beendeveloped for the production of structured resists, wherein two basicgroups are distinguished.

[0004] In the case of positive photoresists, the exposed parts areremoved in the development step and form trenches in the structuredphotoresist while the unexposed parts remain on the substrate and formthe lands or webs of the structured resist. In the case of negativeresists, in contrast to the positive-working resists, the exposed partof the resist remains on the substrate while the unexposed part isremoved by the developer solution. The difference between the solubilityof exposed and unexposed photoresists can be achieved, for example, byinitiating, with an exposure, a chemical reaction in the negativephotoresist such that the photoresist is crosslinked and thus becomesinsoluble.

[0005] In order to achieve a high quantum yield and an adequate processspeed, as a rule chemically amplified resist materials are used. A photoacid generator or a photo base generator which exhibits a high quantumyield at the exposure wavelength has been added to the resist materials.In the case of positive-working photoresists, the polymer of thephotoresist has, for example, acid-labile groups. The exposure liberatesa very strong acid, through the use of which the acid-labile groups arecatalytically eliminated in a baking step. Through the elimination ofthe acid-labile group, a polar group is liberated, for example acarboxyl group or a hydroxyl group, with the result that the polarity ofthe polymer is increased and the latter becomes soluble in aqueousalkaline developers. After the development, the structures produced aretransferred to the substrate by etching processes. In the case of dryetching, the etching process is generally carried out using a fluorineor oxygen plasma. In order to be able selectively to etch only the bareparts of the substrate, the structured resist must have sufficientresistance to the plasma used. The structured resist must thereforeeither have a sufficiently large layer thickness or groups whichincrease the etching resistance of the polymer are provided in thepolymer. When an oxygen-containing etching plasma is used, thephotoresist generally contains silicon-containing groups. These areconverted during the etching process into silicon dioxide, which forms aprotective layer stable to etching on the photoresist. In the case of afluorine plasma, the use of silicon-containing polymers is not expedientsince the silicon is converted into volatile silicon tetrafluoride underthe action of the fluorine plasma. In this case, polymers which have ahigh content of aromatic or alicyclic groups are therefore used.

[0006] The advances made in semiconductor technology require theresolution of smaller and smaller structures. In order to overcome thelimits set by the resolution of the photolithography techniquescustomary at present, attempts are being made to use radiation ofincreasingly short wavelengths for exposure of the photoresist. Fluorinelasers which emit light having a wavelength of 157 nm are favored as themost promising light source. However, the currently used resistmaterials, which are exposed to light having a wavelength of 248 or 193nm, do not have sufficient transparency at a wavelength of 157 nm. Inorder to ensure that sufficient quantities of light penetrate even intothe deep layers of the photoresist, only extremely thin layers cantherefore be used. In order to achieve sufficient etching resistance,the structured resist must therefore be chemically amplified in aseparate step after the development. The chemical amplificationintroduces into the polymer groups which increase the etching resistanceof the resist.

[0007] In addition to sufficient transparency, the polymer must alsohave adequate film formation properties, a sufficiently high softeningtemperature and sufficient adhesion to the substrate. In order to attainthese properties, it is necessary to use high molecular weight polymers.The polymers used for photoresists have to date generally been producedby free radical polymerization and therefore have a backbone of carbonatoms. If a polymer molecule is arranged in such a way that it extendsacross the boundary line between exposed and unexposed parts of thephotoresist, the acid-labile groups are eliminated only in the exposedsegment of the polymer molecule on exposure and baking. Consequently,the polarity of the total polymer chain changes only gradually, so thatthe total polymer chain is removed from the surface of the substratefrom a certain threshold value in the change of the polarity of thepolymer in the developing step. This causes poorly defined lines in thestructure produced by exposure. With decreasing dimensions of thestructures produced, difficulties with the resolution of the structureare therefore encountered. The poor line definition should not exceed avalue of about 5 nm. However, the dimensions of the currently usedpolymers exceeds this value by far. In order to improve the resolution,it is therefore necessary to use shorter polymer chains in classicalresists. However, this causes a deterioration in the further materialproperties, such as film formation or glass transition temperature.

[0008] In order to reduce the poor line definition, it has been proposedto provide acid-labile groups in the main chain of the polymer. In thebaking step following the exposure, the acid-labile groups are cleaved.This leads to depolymerization of the polymer main chain in the exposedparts, with the result that fragments of the polymer are produced. Smallmolecules having completely different properties, for example withrespect to polarity, molecular weight or solubility, are thus produced.Although the poor line definition can be reduced by using thesepolymers, the photoresists still have insufficient transparency at shortwavelengths.

SUMMARY OF THE INVENTION

[0009] It is accordingly an object of the invention to provide aphotoresist which overcomes the above-mentioned disadvantages of theheretofore-known photoresists of this general type and which has a hightransparency to light of short wavelengths, in particular a wavelengthof 157 nm, and which allows to produce structures in an exposure stepthat have well defined lines.

[0010] With the foregoing and other objects in view there is provided,in accordance with the invention, a photoresist including:

[0011] a polymer which is obtained by polycondensation of at least onemonomer and which has acid-cleavable groups in its main chain, at leastone of the monomers having one or more fluorine or fluoroalkyl groups;

[0012] a photo acid generator; and

[0013] a solvent.

[0014] In contrast to the polymers used to date for photoresists andhaving a backbone of carbon atoms, the photoresist according to theinvention includes a polymer which contains acid-cleavable groups in itsmain chain which forms the backbone of the polymer. It is thereforepossible to use high molecular weight polymers which have good filmformation properties, good adhesion to the substrate and a sufficientlyhigh glass transition temperature. Since the polymer is cleaved by theacid liberated from the photo acid generator during exposure, smallerfragments are formed which can then be removed from the developer orremoved in a dry development step. In contrast to the polymers used todate, no partly modified polymer chains are therefore developed awayfrom the partially exposed parts, but only the polymer fragments in theexposed parts. This permits a substantial reduction in the poor linedefinition and hence the production of smaller structures by exposure.Difficulties caused by the low transparency of the known polymers atshort wavelengths are overcome by at least partial fluorination of thepolymer. The polymer is therefore prepared in a condensation reactionfrom at least one monomer which is at least partly fluorinated. After atleast one of the monomers has been completely or partly fluorinated, thepolymer has sufficient transparency even to light of short wavelength.The layer thicknesses of the photoresist of up to 200 nm are thuspossible.

[0015] The photo acid generators customary for photoresists can be usedas photo acid generators. Onium compounds, as described, for example, inPublished European Patent Application No. EP 0 955 562 A1, arepreferably used.

[0016] All customary solvents or mixtures thereof which are capable oftaking up the components of the photoresist in a clear, homogeneoussolution having a long shelf life and which ensure a good layer qualityduring coating of the substrate can be used as solvents.

[0017] The photoresist is applied to the substrate by customary methods,for example by spin-coating, spraying on or immersion methods. Thesolvent is, then removed by customary methods. For this purpose, thesubstrate with the resist film is generally heated.

[0018] If the polymer obtained by polycondensation is prepared from onlya single monomer, the monomer must be at least bifunctional, i.e. musthave at least one nucleophilic substituent and at least one activatedsubstituent, so that a bond is formed by a nucleophilic attack withcondensation.

[0019] Suitable monomers are, for example, selected from a groupincluding aromatic, aliphatic and cycloaliphatic hydroxycarboxylic acidsand their activated derivatives, hydroxyl-substituted isocyanates andactivated hydroxyl-substituted carbonic acid derivatives.

[0020] According to a particularly preferred embodiment, the polymer isprepared from at least a first and a second monomer. In this case,monomers which carry only nucleophilic or only activated substituentscan be used. The first monomer may then include, for example, at leasttwo nucleophilic substituents and the second monomer at least twoactivated substituents which can be subjected to nucleophilic attack.

[0021] The first monomer which is used for the preparation of thepolymer is preferably selected from the group including dicarboxylicacids and their activated derivatives, diisocyanates and activatedcarbonic acid derivatives. Activated dicarboxylic acids are understoodas meaning those derivatives which have high reactivity with respect tonucleophilic attack. Examples of such activated dicarboxylic acids areacid chlorides, acid anhydrides and esters. Examples of activatedcarbonic acid derivatives are phosgene or activated dicarbonates, suchas, for example, dicarbonylimidazoles, di-p-nitrophenyl carbonates orbis(trichlorophenyl)carbonates.

[0022] A compound which carries at least two nucleophilic groups is usedas a reactant of the first monomer. The second monomer is particularlypreferably selected from the group including diols and diamines.

[0023] As a result of the reaction of the abovementioned first andsecond monomers, different types of polymers can be prepared. Polyestersare obtainable by the reaction of dicarboxylic acids with diols, whilepolyamides are obtained by the reaction with diamines. Polyurethanes andpolyureas can be prepared by the reaction of diisocyanates with diolsand diamines, respectively. Polycarbonates and likewise polyurethanesare obtained from activated carbonic acid derivatives. Analogouscompounds can also be obtained from corresponding monomers which are atleast bifunctionally substituted, i.e. include a nucleophilicsubstituent and a substituent which can be subjected to nucleophilicattack.

[0024] The synthesis of polycarbonates using carbonyldiimidazole isdescribed below. The radical R^(x) represents an alkyl, aryl orcycloalkyl radical which may also be completely or partly fluorinated orsubstituted by fluoroalkyl groups. In order to modify the properties ofthe polymer, mixtures of different monomers can also be used for thepolycondensation.

[0025] The polymer is particularly preferably polycarbonate or apolyester.

[0026] According to the invention, at least one of the monomers used forthe preparation of the polymer is completely or partly fluorinatedand/or carries one or more fluoroalkyl substituents, in order toincrease the transparency of the polymer to light of short wavelength.

[0027] For the preparation of at least partly fluorinated polyesters orpolyamides, the first monomer is preferably selected from the groupincluding tetrafluoroterephthalic acid, tetrafluorophthalic acid,terefluoroisophthalic acid and activated derivatives thereof. Forexample, an acid chloride can be used as the activated derivative.

[0028] For the preparation of polycarbonates, bis(trifluoroisopropanol)-and bis(hexafluoroisopropanol)-substituted aliphatic, cycloaliphatic andaromatic compounds are particularly preferred, at least one of thehydroxyl groups being in the form of an activated carbonic acid ester.For example, a dicarbonylimidazole can be used as the activatedderivative.

[0029] If a diol is used as the second monomer, this is preferablylikewise completely or partly fluorinated and therefore has one or morefluoroalkyl substituents.

[0030] If the polymer is in the form of a polyester or polycarbonate,the alcohol component, preferably a diol, preferably has a compositionsuch that at least one of the hydroxyl groups of the alcohol component,in particular of the diol, is bonded to a tertiary carbon atom and atleast one further carbon atom which carries at least one hydrogen atomis bonded to the tertiary carbon atom. The carbonate or ester groups inthe main chain of the polymer can then be very easily cleaved under thecatalytic action of acid, it being possible to increase the reactionrate of the cleavage of the main chain of the polymer by heating theexposed photoresist in a baking step following the exposure. Thecleavage of the main chain is effected according to the followingreaction scheme, only the essential segments in the main chain of thepolymer being shown for simplicity.

[0031] Particularly preferred second monomers are represented by formulaI.

[0032] There, R^(a) represents a divalent alkyl radical, a divalentcycloalkyl radical, a divalent phenyl or biphenyl radical, or

[0033] wherein it is also possible for one or more hydrogen atoms in theradical R^(a) to be replaced by a fluorine atom or a fluoroalkyl group,and X^(a), X^(b), X^(c) and X^(d), in each case independently of oneanother, represent a hydrogen or a fluorine atom.

[0034] The alkyl radical preferably includes 1 to 10 carbon atoms, thecycloalkyl radical 5 to 20 carbon atoms and the fluoroalkyl group of theradical R^(a) preferably 1 to 10 carbon atoms.

[0035] Particularly preferred second monomers are shown below:

[0036] First monomers which carry activated substituents can be producedfrom the diols. The first monomers can be represented by the followinggeneral formulae:

[0037] X^(a), X^(b), X^(c) and X^(d), in each case independently of oneanother, representing a hydrogen or fluorine atom, and A representing anactivating group. A may be, for example:

[0038] Particularly preferred asymmetrical monomers are shown below:

[0039] X^(a), X^(b), X^(c), X^(d) and A in each case having theabove-mentioned meaning.

[0040] Owing to the phenyl group or the cyclohexyl group, these monomersimpart high etching resistance to the polymer. Among the diols shown,those which carry a methyl group on at least one of the tertiary carbonatoms are particularly preferred.

[0041] In addition to the diols shown, triols and their activatedderivatives, preferably bisactivated triols, can also be used as a firstor second monomer. The reaction is preferably carried out in such a waythat only two of the hydroxyl groups are contained in the main chain ofthe polymer, i.e. the polymer is linear. The third hydroxyl groupincreases the solubility of the cleavage products in aqueous polardevelopers in the exposed parts. In the unexposed parts, the thirdhydroxyl group is then available as an anchor group for furthermodification of the polymer, i.e. in an amplification reaction.Preferred monomers or triols which can be used as precursors of monomersare represented below by the general formula II.

[0042] R^(b) representing a trivalent alkyl radical, a trivalentcycloalkyl radical or a trivalent phenyl or biphenyl radical, or

[0043] wherein it is also possible for one or more hydrogen atoms in theradical R^(b) to be replaced by a fluorine atom or a fluoroalkyl group.

[0044] The alkyl radical preferably includes 1 to 10 carbon atoms, thecycloalkyl radical 5 to 20 carbon atoms and the fluoroalkyl group of theradical R^(a) preferably 1 to 10 carbon atoms. X^(a), X^(b), X^(c),X^(d) X^(e) and X^(f), in each case independently of one another,represent a hydrogen or fluorine atom, and Y¹, Y² and Y³, independentlyof one another, represent a hydrogen atom or an activated carbonic acidester. Y¹, Y² and Y³ preferably have a structure

[0045] A being an activating substituent, in particular

[0046] The bisactivated carbon acid derivatives of the triols areparticularly preferred.

[0047] Particularly preferred triols are shown below:

[0048] The activated carbonic acid derivatives are obtained by reactingone, two or three of the hydroxyl groups with a corresponding activatedcompound, e.g. carbonyldiimidazole. Preferably, two of the hydroxylgroups are converted into an activated carbonic acid derivative.

[0049] The cleavage of the main chain of the polymer gives rise to polargroups, so that the cleavage products already have sufficient solubilityin the aqueous developer. Owing to its modular structure, the polymercontained in the photoresist according to the invention can be veryexactly adapted to the requirements which are set for the photoresist.For this purpose, further comonomers can be incorporated into thepolymer, in addition to the abovementioned monomers. The cleavage of themain chain may therefore also give rise to cleavage products whosesolubility in the developer is no longer sufficient for achieving aclear differentiation between exposed and unexposed parts of thephotoresist in acceptable processing times. In these cases, it may beadvantageous if comonomers which additionally have acid-labile sidegroups are incorporated into the polymer by condensation. During theelimination of the acid-labile group, which generally takes placesimultaneously with the cleavage of the main chain, further polargroups, such as carboxyl groups or acidic hydroxyl groups, are thenliberated, with the result that the solubility of the cleavage productsin aqueous alkaline developers is increased. In this embodiment, thepolymer therefore has additional acid-labile side groups which, aftertheir elimination, liberate a polar group.

[0050] The groups known from positive photoresists can be used asacid-labile groups. Examples of suitable acid-labile groups are:tert-alkyl ester, tert-butoxycarbonyloxy, tetrahydrofuranyl,tetrahydropyranyl, tert-butyl ether, lactone or acetal groups.Tert-Butyl ester groups are particularly preferred. Exemplary comonomersare represented by the formula III:

[0051] R having the meaning stated in formula II, W representing —COOR′,—OR′, —OCOOR′, and

[0052] R′ representing —C^(n)H_(2n+1), with n=1-10.

[0053] Examples of particularly preferred monomers are shown below:

[0054] Here, X^(a), X^(b), X^(c), X^(d), X^(e), X^(f) and X^(g), in eachcase independently of one another, represent a hydrogen or fluorineatom.

[0055] After the structuring of the photoresist, the structure producedis transferred to the substrate underneath by dry etching. The etchingresistance of the resist must therefore be tailored to the plasma used.If an oxygen plasma is used, the polymer preferably includes monomerunits which have silicon-containing groups. These may already be presentin the polymer or may be subsequently linked to corresponding anchorgroups in the polymer. The customary processes for chemicalamplification, as described, for example, in Published European PatentApplication No. EP 0 395 917 B1, may be used for this purpose. Monomericcompounds through the use of which silicon-containing groups can bedirectly incorporated into the polymer chain are shown below.

[0056] R^(v) representing —CH₃, —C₆H₅ or —C₆H₁₁, and o representing aninteger between 0 and 50.

[0057] In order to increase the etching resistance of the polymer to afluorine plasma, for example, an alicyclic or aromatic monomer can beincorporated into the polymer. Suitable alicyclic monomers are, forexample, fluorine- or fluoroalkyl-substituted difunctional norbornanederivatives or adamantane compounds.

[0058] Further repeating units of the polymer are shown below:

[0059] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0060] Although the invention is illustrated and described herein asembodied in a resist based on polycondensates and having an increasedresolution for use in 157 nm lithography, it is nevertheless notintended to be limited to the details shown, since various modificationsand structural changes may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

[0061] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] The invention is explained in more detail with reference to anexemplary embodiment.

[0063] The polymer for the photoresist according to the invention isobtained by cocondensation of bis(hydroxymethyl)tricyclo [5.2.1.0^(2.6)]decane (10.0 g, 0.051 mol), oxalyl chloride (6.466 g, 0.051 mol),tetrafluoroterephthaloyl chloride (14.024 g, 0.051 mol) and2,5-dihydroxy-2,5-di(trifluoromethyl)hexane (12.963 g, 0.051 mol). Forthis purpose, the acid chlorides are initially introduced as a solutionin tetrahydrofuran (50 ml) and a solution of the alcohols intetrahydrofuran/pyridine is added dropwise so that the temperature ofthe mixture does not exceed 10° C. Stirring is carried out for 12 hoursat room temperature. The precipitate is separated off and the filtrateis introduced into methanol (1000 ml). A white precipitate forms. Thisis separated off and is further purified by reprecipitation three timesfrom tetrahydrofuran/methanol. After drying, 27.54 g of white polymerare obtained.

[0064] The structure of the polymer is shown below. Here, p represents anumber between 10 and 100.

We claim:
 1. A photoresist, comprising: a polymer obtained bypolycondensation of at least one monomer, said polymer having a mainchain and having acid-cleavable groups in said main chain, said at leastone monomer having at least one group selected from the group consistingof a fluorine group and a fluoroalkyl group; a photo acid generator; anda solvent.
 2. The photoresist according to claim 1, wherein said polymeris composed of a monomer carrying at least one nucleophilic substituentand at least one activated substituent.
 3. The photoresist according toclaim 1, wherein said at least one monomer is a monomer selected fromthe group consisting of an aromatic hydroxycarboxylic acid, an aliphatichydroxycarboxylic acid, a cycloaliphatic hydroxycarboxylic acid, anactivated derivative of an aromatic hydroxycarboxylic acid, an activatedderivative of an aliphatic hydroxycarboxylic acid, an activatedderivative of a cycloaliphatic hydroxycarboxylic acid, ahydroxyl-substituted isocyanate, and an activated hydroxyl-substitutedcarbonic acid derivative.
 4. The photoresist according to claim 1,wherein said polymer is composed of at least a first monomer and asecond monomer.
 5. The photoresist according to claim 4, wherein saidfirst monomer is selected from the group consisting of a dicarboxylicacid, an activated derivative of a dicarboxylic acid, a diisocyanate andan activated carbonic acid derivative.
 6. The photoresist according toclaim 4, wherein said second monomer is a monomer selected from thegroup consisting of a diol and a diamine.
 7. The photoresist accordingto claim 1, wherein said polymer is a polymer selected from the groupconsisting of a polyester and a polycarbonate.
 8. The photoresistaccording to claim 5, wherein said dicarboxylic acid is an acid selectedfrom the group consisting of tetrafluoroterephthalic acid, atetrafluorophthalic acid and a tetrafluoroisophthalic acid.
 9. Thephotoresist according to claim 4, wherein said first monomer is amonomer selected from the group consisting of an activated carbonic acidderivative of a bis(trifluoroisopropanol)-substituted aromatic, abis(hexafluoroisopropanol)-substituted aromatic, an aliphatic, and acycloaliphatic.
 10. The photoresist according to claim 6, wherein saiddiol has at least one group selected from the group consisting of afluorine group and a fluoroalkyl group.
 11. The photoresist according toclaim 4, wherein: at least one of said first and second monomers is adiol including hydroxyl groups; a tertiary carbon atom is bonded to atleast one of said hydroxyl groups; and at least a further carbon atom isbonded to said tertiary carbon atom, said further carbon atom carries atleast one hydrogen atom which can be eliminated.
 12. The photoresistaccording to claim 4, wherein said second monomer has a structureaccording to formula I:

R represents a radical selected from the group consisting of a divalentalkyl radical, a divalent cycloalkyl radical, a divalent phenyl radical,a divalent biphenyl radical, and

and X^(a), X^(b), X^(c) and X^(d), in each case independently of oneanother, represent one of a hydrogen atom and a fluorine atom.
 13. Thephotoresist according to claim 12, wherein said radical R has at leastone hydrogen atom replaced by one of a fluorine atom and a fluoroalkylgroup.
 14. The photoresist according to claim 4, wherein said firstmonomer is an activated carbonic acid derivative of a diol structureaccording to formula I:


15. The photoresist according to claim 1, wherein said at least onemonomer is a monoactivated, carbonic acid derivative of a diol structureaccording to formula I:


16. The photoresist according to claim 4, wherein at least one of saidthe first monomer and said second monomer has a structure according toformula I:

R represents a radical selected from the group consisting of a trivalentalkyl radical, a trivalent cycloalkyl radical, a trivalent phenylradical, a trivalent biphenyl radical, and

X^(a), X^(b), X^(c) X^(d), X^(e) and X^(f), in each case independentlyof one another, represent one of a hydrogen atom and a fluorine atom,and Y¹, Y² and Y³, independently of one another, represent one of ahydrogen atom and an activated carbonic acid ester.
 17. The photoresistaccording to claim 16, wherein said radical R has at least one hydrogenatom replaced by one of a fluorine atom and a fluoroalkyl group.
 18. Thephotoresist according to claim 4, wherein at least one of said the firstmonomer and said second monomer has a structure according to formula I:

R represents a radical selected from the group consisting of a trivalentalkyl radical, a trivalent cycloalkyl radical, a trivalent phenylradical, a trivalent biphenyl radical, and

X^(a), X^(b), X^(c) X^(d), X^(e) and X^(f), in each case independentlyof one another, represent one of a hydrogen atom and a fluorine atom,and Y¹, Y² and Y³ have a structure according to formula II:

wherein A is an activating substituent.
 19. The photoresist according toclaim 18, wherein said activating substituent has a structure accordingto formula III:


20. The photoresist according to claim 4, wherein said first monomer isa bisactivated carbonic acid derivative of a structure according toformula I:

R represents a radical, X^(a), X^(b), X^(c) X^(d), X^(e) and X^(f), ineach case independently of one another, represent one of a hydrogen atomand a fluorine atom, and Y¹, Y² and Y³ have a structure according toformula II:

wherein A is an activating substituent.
 21. The photoresist according toclaim 1, wherein said polymer has acid-labile side groups such that saidacid-labile side groups liberate a polar group when being eliminated.22. The photoresist according to claim 1, wherein said polymer hassilicon-containing groups.